A major quality consideration for the production of sheet materials, such as paper, is strength. Until recently, all strength measurements with respect to such sheet materials were made by off-line laboratory measurements. Recently, on-line measurements have been introduced using contacting gauging techniques that rely on the relationship between Young's Modulus and the speed of sound according to the following equation: EQU Y=K.sub.1 S.sup.2
Where k.sub.1 is a function of the density of the material and s is the speed of sound within the material.
The methods of Baum and Habeger, as set forth in U.S. Pat. No. 4,291,577, and others, rely on rotating wheels which contact the moving web of paper or other material whose strength is being measured. The wheels contain piezoelectric or magnetostrictive transducers in their outer peripheries to create a localized contraction and expansion in the moving web of material. This contraction and expansion creates a sonic wave that travels radially from the spot of creation. Measuring the speed of sound within the material, which is the reciprocal of the transit time between two points of known separation, is used in conjunction with the density of the material to provide a measurement of the strength of the material. This approach has some inherent disadvantages among which are that the required commutation and mechanical contact produce a signal that contains a significant amount of noise, the rotating wheels are prone to fail, mechanical structures are inevitably more costly and have more parts than electronic devices, the direct contact of the wheels with the material limits the measurement of strength to a single direction (either across the web or along the web), and mechanical methods with slippage and commutation are inherently less accurate than non-mechanical methods.
Photoacoustic interaction has been used to induce ultrasonic waves into a continuous, fast moving web of paper. U.S. Pat. No. 4,674,332 (Pace, et al) discloses the use of a nitrogen laser to illuminate paper with high power ultraviolet pulses. A portion of this optical energy is converted into heat creating an acoustic wave from the resulting thermal expansion. A contacting, ultrasonic sensor or a microphone positioned on the opposite side of the paper receives the acoustic wave and provides an indication of the speed of sound through the paper which can be utilized to determine the strength of the paper in its direction of movement.
Another application of a laser to generate acoustic waves in paper is provided in U.S. Pat. No. 4,622,853 (Leugers). The apparatus disclosed in this reference utilizes a Neodymium/Yttrium-Aluminum-Garnet (Nd/YAG) laser with a frequency doubler to illuminate a spot on a moving web of paper. The ultrasonic wave in the paper is detected by an ultrasonic transducer in contact with the paper.
Because of the disadvantages that are inherent in a measuring system that requires contact with the material whose strength is being measured, it has become desirable to develop an on-line, measuring system that does not require such contact.